Cold cathode fluorescent lamp low dimming antiflicker control circuit

Abstract

A flicker reduction method for a lamp assembly includes providing current pulses to a fluorescent lamp in response to a periodic signal such as a ramp voltage. The current pulses illuminate the fluorescent lamp. The method further includes providing at least a predetermined number of current pulses to the fluorescent lamp per period of the periodic signal. A feedback circuit samples current in the fluorescent lamp to ensure that a predetermined number of current pulses have been provided. After the pulses are provided, the circuit is reset for the next cycle of the periodic signal.

I claim:  
     
       1. A method of eliminating flicker for a lamp assembly, the method comprising: 
       generating a substantially periodic ramp voltage signal;  
       in response to the substantially periodic ramp voltage signal, providing current pulses to a fluorescent lamp to illuminate the fluorescent lamp; and  
       in response to an indication of current in the fluorescent lamp, providing at least a predetermined number of current pulses per period of the substantially periodic ramp voltage signal to the fluorescent lamp.  
     
     
       2. The method of  claim 1  further comprising: 
       maintaining the current in the fluorescent lamp above a predetermined current threshold by providing at least the predetermined number of current pulses per time period.  
     
     
       3. A method of eliminating flicker for a lamp assembly, the method comprising: 
       generating a substantially periodic ramp voltage signal;  
       in response to the substantially periodic ramp voltage signal, providing current pulses to a fluorescent lamp to illuminate the fluorescent lamp;  
       in response to an indication of current in the fluorescent lamp, providing at least a predetermined number of current pulses per period of the substantially periodic ramp voltage signal to the fluorescent lamp;  
       receiving a dimming level input signal;  
       varying the current pulses provided to the fluorescent lamp in response to level of the dimming level input signal;  
       detecting current pulses in the fluorescent lamp; and  
       when a detected number of current pulses in the fluorescent lamp falls below a predetermined current threshold, providing at least the predetermined number of current pulses per period of the periodic signal to the fluorescent lamp independent of the level of the dimming level input signal.  
     
     
       4. The method of  claim 3  further comprising: 
       comparing the dimming level input signal to the substantially periodic ramp voltage signal;  
       providing the current pulses to the fluorescent lamp when the dimming level input signal exceeds a threshold related to amplitude of the substantially periodic ramp voltage signal; and  
       in response to the detected current in the fluorescent lamp, clamping the dimming level input signal at a level at least equal to a minimum value of the substantially periodic ramp voltage signal to provide at least the predetermined number of current pulses per period of the substantially periodic ramp voltage signal.  
     
     
       5. The method of  claim 4  further comprising: 
       varying the threshold to a recover voltage following a predetermined time to prevent provision of additional current pulses during a present period of the periodic ramp voltage signal.  
     
     
       6. The method of  claim 5  further comprising: 
       resetting the threshold to an initial value for each period of the periodic ramp voltage signal.  
     
     
       7. The method of  claim 6  further comprising: 
       detecting the periodic ramp voltage signal; and  
       when the periodic ramp voltage signal is less than a reset value, providing additional current pulses to the fluorescent lamp.  
     
     
       8. A control circuit for a fluorescent lamp, the control circuit comprising: 
       a controller generating control signals in response to a received brightness control signal;  
       an amplifying circuit coupled to the controller and the fluorescent lamp to provide current pulses at an output in response to the control signals at an input of the amplifying circuit;  
       a comparator which compares a dimming level input signal and a time varying signal to produce the brightness control signal; and  
       a feedback circuit which detects current in the fluorescent lamp to control the dimming level input signal at predetermined operating conditions of the control circuit.  
     
     
       9. The control circuit of  claim 8  wherein the amplifying circuit comprises: 
       a transformer having a primary winding to receive control signals and a secondary winding to provide the current pulses in response to the control signals.  
     
     
       10. The control circuit of  claim 8  wherein the feedback circuit is configured to maintain the dimming level input signal at a voltage level for a time sufficient to ensure provision of a minimum number of current pulses before reducing the dimming level input signal to a level less than the time varying signal to interrupt further current pulses. 
     
     
       11. The control circuit of  claim 10  wherein the feedback circuit comprises: 
       a comparator which compares a voltage related to the current in the fluorescent lamp and a threshold voltage having an initial value related to a value of the dimming level input signal.  
     
     
       12. The control circuit of  claim 11  further comprising: 
       biasing circuitry which varies the threshold voltage between the initial value of the threshold voltage and a recover value of the threshold voltage.  
     
     
       13. A method for operating a fluorescent lamp, the method comprising: 
       producing a ramp voltage waveform;  
       receiving a dimming level input signal associated with a desired dimming level for the fluorescent lamp;  
       comparing the ramp voltage waveform and the dimming level input signal;  
       generating current pulses for the fluorescent lamp when the ramp voltage waveform is less than the dimming level input signal;  
       detecting fluorescent lamp current; and  
       in response to the detected fluorescent lamp current, clamping the dimming level input signal at a minimum value sufficient to eliminate flicker in the fluorescent lamp at desired operating conditions.  
     
     
       14. The method of  claim 13  wherein clamping the dimming level input signal comprises: 
       charging a storage device in response to the detected fluorescent lamp current;  
       comparing voltage on the storage device with a threshold voltage; and  
       when the voltage on the storage device exceeds the threshold voltage, releasing the clamping of the dimming level input signal.  
     
     
       15. The method of  claim 14  wherein releasing the clamping of the dimming level input signal comprises: 
       resetting an output of a comparator to a voltage below the dimming level in put signal;  
       varying the threshold voltage to a recover voltage; and  
       in the comparator, comparing the voltage on the charge storage device and the recover voltage.  
     
     
       16. A control circuit for a fluorescent lamp, the control circuit configured to receive a dimming level input signal and to produce a ramp voltage wave form and current pulses when the ramp voltage wave form is less than the dimming level input signal by comparing the ramp voltage wave form and the dimming level input signal in a comparator, the control circuit comprising: 
       a feedback comparator configured to compare a signal related to current in the fluorescent lamp and a variable threshold signal to produce a control signal which controls the feedback comparator to ensure at least a predetermined number of current pulses are provided to the fluorescent lamp during each period of the ramp voltage wave form at a desired operating condition.  
     
     
       17. The improved control circuit of  claim 16  wherein the feedback comparator is configured to maintain at least the predetermined number of current pulses to the fluorescent lamp independent of operating temperature. 
     
     
       18. A method for operating a fluorescent lamp, the method comprising: 
       providing current pulses to the fluorescent lamp;  
       monitoring lamp current in the lamp; and  
       in response to the lamp current, adjusting number of current pulses to the fluorescent lamp to substantially eliminate visible flicker of the fluorescent lamp.  
     
     
       19. The method of  claim 18  further comprising: 
       in response to the lamp current, maintaining a minimum number of current pulses per time period independently of operating temperature of the fluorescent lamp.  
     
     
       20. A controller for a fluorescent lamp comprising: 
       an input circuit which receives a dimming level input signal;  
       a circuit which provides a variable number of current pulses per time period to the fluorescent lamp, the number of current pulses being related to value of the dimming level input signal; and  
       an input override circuit configured to override the dimming level input signal to ensure that at least a minimum number of current pulses per time period are provided to the fluorescent lamp.  
     
     
       21. A method for operating a fluorescent lamp, the method comprising: 
       providing current pulses to the fluorescent lamp during duration of a pulse-width modulation signal to illuminate the fluorescent lamp;  
       interrupting provision of the current pulses when a threshold is exceeded;  
       suspending provision of subsequent current pulses during a recovery time; and  
       after the recovery time, again providing the current pulses to the fluorescent lamp.  
     
     
       22. The method of  claim 21  further comprising: 
       ensuring a minimum number of current pulses are provided before interrupting provision of the current pulses to substantially eliminate visible flicker of the fluorescent lamp.  
     
     
       23. A method for operating a fluorescent lamp, the method comprising: 
       providing current pulses to the fluorescent lamp during duration of a pulse-width modulation signal to illuminate the fluorescent lamp;  
       in response to the current pulses, generating a stair step signal indicative of number of pulses provided;  
       comparing the stair step signal with a threshold value; and  
       interrupting provision of the current pulses when the threshold is exceeded by the stair step signal.  
     
     
       24. The method of  claim 23  further comprising: 
       ensuring a minimum number of current pulses are provided before interrupting provision of the current pulses to substantially eliminate visible flicker of the fluorescent lamp.